Variable reactance circuits



y 5, 1955 R. w. JONES 2,712,637

VARIABLE REACTANCE CIRCUITS Filed April 25, 1951 IN VEN TOR. Robert It Jane's Attorneys atent 2,712,637 Patented July 5, 1955 VARIABLE REACTANCE CIRCUITS Robert W. Jones, United States Navy Appiication April 25, 1951, Serial No. 222,922

2 Claims. (Cl. 33228) (Granted under Title 35, U. S. Code (1952), see. 266} This invention relates to variable reactance circuits and particularly to a variable reactance circuit which can be used in frequency-shift keying an oscillator or as a frequency modulator of an oscillator.

An object of the invention is to provide a variable reactance circuit of simple and relatively inexpensive construction which can be used to modulate, for example, a radio frequency oscillator at an audio rate while insuring substantial isolation of the audio voltages from the radio frequency voltages in the oscillator.

In general, the invention comprises an electron discharge device such as a triode that is operated in a certain manner in a circuit so as to exhibit a variable reactance between its grid and cathode to any circuit, such as the tank circuit of a radio frequency oscillator, connected between these points. This reactance is varied by taking advantage of the Miller efiect, the change in grid-cathode capacity resulting from a change in amplification factor. The amplification factor is conveniently changed by changing the anode voltage on the triode by keying or by modulating with a signal a second triode whose anode current is used to control the anode voltage of the first triode. Although similar variable reactance circuits are known in the prior art and have been based on somewhat the same principles, the present invention insures substantial isolation of the signal voltages from the oscillator voltages by preventing substantially any coupling between the anode and grid of the first triode. In effect, the second tube constitutes a variable resistance connected between the anode and cathode return of the first tube.

Reference is now made to the drawing in which the single figure shows a simple form of circuit made in accordance with the invention. Numeral 1 indicates generally the tank circuit or frequency-determining circuit of a radio frequency oscillator, for example, which it is desired to key or modulate by connections to terminals 2 and 3. The keying or modulation is accomplished by the use of an electron discharge device exemplified by the triode 4 controlled by another triode 5. These are illustrated as being enclosed in the same envelope and constituting a duplex triode but, of course, could be separate tubes. The variable reactance of the novel circuit appears between the grid 6 and the cathode return 7 of the triode 4 across the terminals 8 and 9 and conveniently can be applied to the oscillator 1 through a coupling condenser 14). The anodes 11 and 12 of the tubes 4 and 5, respectively, are directly connected and supplied through a common resistor 13 with a source of direct current potential, usually called B supply, connected between terminals B and B+. The grid 6 of the tube 4 is provided with a grid resistor 14 while its cathode 15 is provided with a cathode bias resistor 16. Similarly the grid 17 of tube 5 is provided with a grid resistor 18 while its cathode 19 is provided with a cathode bias resistor 20 preferably in the form of a potentiometer, part of which can be shorted out by a key 21. A source of signal, such as an audio signal generator, microphone or the like, is indicated generally at 22 and is adapted to be connected at 23 across the grid resistor 18. Radio frequency by-pass condensers 24, 25, and 26 are included for proper operation.

The operation of the device and the method by which the triode 4 is used to provide a variable reactance will be clear from the following. If the anode voltage on the anode 11 is altered the amplification factor of the tube 4 is altered producing a corresponding variation in the capacitance between the grid 6 and the cathode 15. This anode voltage can be varied by varying the voltage drop in the common resistor 13 which in turn can be accomplished by varying the effective resistance of the tube 5 that constitutes, in efiect, a resistance connected between the anode and cathode return of the tube 4. A variation in the elfective resistance of the tube 5 produces a corresponding change in its anode current that causes the desired change in the voltage drop across the common resistor 13. The'efiective resistance of the tube 5 can conveniently be varied in either of two ways. When frequen s -shift keying of the oscillator 1 is desired, the key 21 is operated to vary the magnitude of the cathode bias resistor 21 The magnitude of the frequency shift can be determined by the setting of the potentiometer 29. This keying can also be accomplished, of course, by altering the bias on the grid 17 and either of these changes could use a battery or other source in place of a change in the value of a bias resistor. For frequency modulation of the oscillator 1 the reactance of the circuit across terminals 8 and 9 can be varied, for example, at an audio rate by applying a signal from the source 22 across the grid resistor 18. The eifective resistance and hence the anode current of the tube 5 will thus be varied in accordance with the signal from source 22 and cathode return of the tube 4. There being substantially no coupling between the anode 11 and the grid 6 of the tube 4 there is obtained excellent isolation of the audio voltages from the radio frequency voltages in the oscillator 1 to be modulated.

It is seen then that the triode 4, located in a circuit providing substantially no coupling between its anode and grid, is operated by varying the magnitude of a resistance, namely the tube 5, connected between its anode and cathode return, so as to produce a correspending variation in the capacitance between its grid 6 and cathode 15 appearing as a variation in impedance across the terminals 8 and 9. When this variable reactance circuit is coupled to a radio frequency oscillator, for example, it provides substantially complete isolation of the modulating signal voltages from the carrier frequency voltages.

Although, as hereinbefore indicated, two separate triodes can be used in this invention, satisfactory results have been obtained with a dual triode designated as a 6SL7GT. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention can be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A system adapted for frequency-shift keying of an oscillator comprising two electronic tubes, each having a cathode, an anode, and a grid, means adapted for the connection of said oscillator between the grid and cathode return of the first of said tubes, a substantially direct connection between the anodes of said tubes, the grids of said two devices being electrically isolated from each other, a common source of direct current potential for the anodes of said tubes, and means including a key for varying the effective resistance of the second of said tubes to vary correspondingly the reactance between the grid and cathode of said first tube, said first tube being arranged so thatsubstantially no coupling exists at audio frequencies between the anode and grid thereof. 7

2. The system of claim 1 wherein the key is connected to vary the cathode bias of the second tube.

References Cited in the file of this patent UNITED STATES PATENTS Rothe July 27, 1937 Briggs Jan. 23, 1945 Born Aug. 26, 1947 Wheeler June 6, 1950 

